Physics 17, Winter 2026
Homework #1
(due Friday, Jan. 9, 11:59pm via Gradescope)
Reading
Serway, Chapter 2.1-2.4
Chapter 3.1-3.4
Problem 1:
The charged muon (µ+) is an elementary particle that has an average lifetime of 2.2 µs at
rest. Muons are created in cascades in the upper atmosphere of the Earth as a result of
the interaction of high-energy cosmic rays. Suppose a muon, created with a speed of
0.998c, is traveling down towards the Earth’s surface. As measured in a frame fixed with
respect to the Earth, how far will this muon travel, on average?
Problem 2:
Calculate the speed of a particle:
a) whose total energy is three times its rest energy, and
b) whose kinetic energy is twice its rest energy.
Problem 3:
An electron having kinetic energy of 2.00 MeV makes a head-on collision with a positron
at rest. (A positron is an antimatter particle that has the same mass as the electron but
opposite charge). In the collision the two particles annihilate each other and two γ-rays of
equal energy are produced. Each γ-ray travels at an angle θ relative to the electron’s
direction of motion. (γ-rays are massless particles of electromagnetic radiation). Find the
energy E, momentum p, and angle of emission θ of the γ-rays.
Problem 4:
The Sun has a luminosity of L = 3.9 x 1026 J/s, a mass of M = 2.0 x 1030 kg, and a radius
of R = 7.0 x108 m. If the Sun converts 0.001 of its mass into energy of its lifetime, how
long can it shine? Compare the average density of the Sun with the density of the Pacific
Ocean. Since the Sun derives its energy from the conversion of hydrogen into helium
and since the Pacific Ocean is full of hydrogen, why does the Sun shine and the Pacific
Ocean does not?
Problem 5:
Consider the metals lithium, beryllium, and mercury, which have work functions of 2.3
eV, 3.9 eV, and 4.5 eV, respectively. If light of wavelength 300 nm is incident on each
of these metals, determine: a) which metals exhibit the photoelectric effect and b) the
maximum kinetic energy for a photoelectron in each case.
Problem 6 (Serway 3.12):
A sodium-vapor lamp has a power output of 10 W. Using 589.3 nm as the average
wavelength of the source, calculate the number of photons emitted per second.